PreviewTwo experiments were performed, aimed at exploring a previously identified right-hand kinematic advantage for grasping movements whose end goal is to bring an item to the mouth. We provide evidence that this advantage for hand-to-mouth grasping movements is 1) nonspecific with respect to a target's edibility and 2) dependent on the concurrent opening of the mouth to accept the transported target

Abstract

Long-train electrical stimulation of the motor and premotor cortices of nonhuman primates can produce either hand-to-mouth or grasp-to-inspect movements, depending on the precise location of stimulation. Furthermore, single-neuron recording studies identify discrete neuronal populations in the inferior parietal and ventral premotor cortices that respond uniquely to either grasp-to-eat or grasp-to-place movements, despite their identical mechanistic requirements. These studies demonstrate that the macaque motor cortex is organized around producing functional, goal-oriented movements, rather than simply fulfilling muscular prerequisites of action. In humans, right-handed hand-to-mouth movements have a unique kinematic signature; smaller maximum grip apertures are produced when grasping to eat than when grasping to place identical targets. This is evidence that the motor cortex in humans is also organized around producing functional movements. However, in both macaques and humans, grasp-to-eat/hand-to-mouth movements have always been elicited using edible targets and have (necessarily) been paired with mouth movement. It is therefore unknown whether the kinematic distinction is a natural result of grasping food and/or is simply attributable to concurrent opening of the mouth while grasping. In experiment 1, we used goal-differentiated grasping tasks, directed toward edible and inedible targets, to show that the unique kinematic signature is present even with inedible targets. In experiment 2, we used the same goal-differentiated grasping tasks, either coupled with or divorced from an open-mouth movement, to show that the signature is not attributable merely to a planned opening of the mouth during the grasp. These results are discussed in relation to the role of hand-to-mouth movements in human development, independently of grasp-to-eat behavior